Improve diff tabbing

[krjackso]

Right now in some places we have diffs like

ZoneDiff
changed_records: [RecordDiff, RecordDiff]

where the RecordDiff is responsible for adding the correct number of tabs to look good when the diff for ZoneDiff is printed. ZoneDiff (the parent) should be responsible for the number of tabs a child diff needs so that these Diff types could be reused without having incorrect tabs.